The reaction tube used as the semiconductor diffusion furnace is commonly a cylindrical silicon carbide substrate having a high purity silicon carbide film formed on the inner surface thereof. In the manufacture of the reaction tube, a high purity silicon carbide film is generally formed on the inner surface of a substrate by a chemical vapor phase deposition (CVD) process including feeding a source gas containing a carbon source and a silicon source to the hollow interior of the substrate, and heating the source gas in vacuum for depositing silicon carbide on the substrate inner surface.
The silicon carbide films deposited by the conventional process, however, are relatively poor in denseness, evenness and homogeneity and can give rise to undesirable phenomena such as failure and separation. More particularly, in forming a silicon carbide film on the inner surface of a long cylindrical substrate as required for the reaction tube adapted to construct a diffusion furnace, the source gas is supplied from one open end of the cylindrical substrate. Then a thicker film can deposit on the inner surface in the vicinity of the source gas feeding site or nearer to the one end. An axial change of reaction conditions along the cylindrical substrate can affect the denseness of the film. It is thus difficult to form a film having high denseness, evenness and homogeneity. Thus the reaction tube having an inner coating formed by such a process, when it is used as a semiconductor diffusion furnace, tends to experience a failure or separation of the coating during subsequent heat treatment of semiconductor. This is quite undesirable because the semiconductor can be contaminated by passage or diffusion of impurities originating from silicon carbide substrates.
One solution to this problem is proposed in Japanese Patent Publication No. 6304/1985. A silicon carbide film forming apparatus is disclosed therein as comprising a cylindrical outer shell, a cylindrical electrode disposed within the shell, and a silicon carbide tube disposed within the electrode. A source gas is fed to the interior of the silicon carbide tube from one end thereof while a band-shaped zone of the electrode interior is heated by a ring-shaped induction heater disposed around the shell for axial motion. By moving the induction heater along the shell, the heated zone or reaction zone is axially moved thereby preventing any change in film thickness and denseness between the one end where the source gas is fed and the other end of the tube.
Also, Japanese Patent Application Kokai No. 72075/1991 discloses a silicon carbide film forming apparatus in which silicon carbide film-forming reaction is carried out while a source gas feed conduit is axially moved through the silicon carbide tube to be coated, thereby preventing any change in thickness and denseness of the resulting film in the axial direction throughout the silicon carbide tube.
In these apparatus, however, the distance between the reaction region and a gas discharge outlet changes and it is difficult to control the temperature distribution and gas compositions (carbon source/silicon source ratio, source gas/carrier gas ratio, etc.) therebetween. Although film thickness control is relatively easy, the change of composition in thickness direction of the deposited film can substantially differ between the gas inlet and outlet.